Diablo is good but not worth doing twice when there are other great rides to do...

If you want a taste of the amazing variety SF has to offer I would suggest spreading your rides out... Mt Diablo one day for sure, but Alpine Dam loop from Fairfax, Mt Hamilton (can throw in Sierra rd loop in the same ride if you want steep), and finally Old La Honda-Stages-Tunitas creek rd are well worthwhile including in your stay.

These were the four rides I selected on my visit to SF last year. Details and photos in my blog linked below.